1. Field of the Invention
The invention relates to an image forming device and, more particularly, to an image forming device that can produce a gray-scale image by using a dither pattern.
2. Description of the Related Art
Conventional image forming devices produce bi-level images by combining on and off dots. A dither pattern, in which a plurality of dots are orderly arranged, is known to provide multi-level gray scale printing and produce an image with smooth gradations of density.
A dither pattern includes a plurality of gray level areas arranged in a predetermined manner. FIG. 13 shows a square dither pattern having three rows and three columns of gray level areas. The dither pattern is divided into the 1st to 9th gray level areas. A plurality of orderly arranged dither patterns constitute a dither matrix. By processing predetermined gray-scale image data in such a dither matrix, a continuous gray-scale image can be produced.
As should be appreciated, the term “gray” applies to both monochrome and color images and to the density of both monochrome and color images.
FIGS. 14A–14J show dot forming states in accordance with the gray level values in the dither patterns or the dither matrix. As shown in FIG. 14A, when the gray level is zero, no dots are formed in any of the gray level areas of the dither pattern. As shown in FIG. 14B, when the gray level is 1, a dot is formed only in the 1st area. As shown in FIG. 14F, when the gray level is 5, a dot is formed in each of the 1st to 5th areas. As shown in FIG. 14J, when the gray level is 9, a dot is formed in each gray level area of the dither pattern.
As described above, the number of dots formed in each dither pattern increases/decreases depending on the corresponding gray level. Accordingly, the shades of gray of an image are determined and, as a result, a continuous gray-scale image can be obtained.
A high-quality image can be created when the dots are continuously formed. On the other hand, when the dot formation is temporarily suspended and then restarted, a poor quality image produced immediately after the suspension is created.
For example, in wire dot printers and ink-jet printers, a pressure at the restart of dot formation after a temporary suspension is lower than a steady pressure attained during continuous dot formation. Thus, the printers fail to produce a high-quality image immediately after the suspension.
Also, in thermal head printers, the temperature at the restart of the dot formation after a temporary suspension is lower than a steady temperature attained during continuous dot formation. Thus, the printers fail to produce a high-quality image immediately after the suspension.
The above-described problems are enhanced in laser printers. FIG. 15 shows a relationship among a laser beam control signal, a laser beam, and an image forming state. When a laser beam control signal is turned on, the intensity of a laser beam does not quickly rise to a threshold value that enables image forming. Instead, the intensity of a laser beam rises gradually to the threshold value. Toner only properly adheres to portions of a photosensitive body that have been exposed to a laser beam of a sufficient intensity beyond the threshold value where clear dots are formed. However, no toner adheres to the photosensitive body before the intensity of the laser beam reaches the threshold value after the laser beam control signal has been turned on. When continuous dot formation is temporarily suspended and then restarted, the intensity of the laser beam is below the threshold value. Thus, clear dots or a high-quality image cannot be produced.
As shown in FIG. 14E, when the gray level is 4, a dot is formed in each of the 1st to 4th gray level areas of the dither pattern. As far as the laser beam scanning direction is concerned, the 4th area is isolated from the 1st to 3rd areas. Prior to forming a dot in the 4th area, dot formation must be temporarily suspended. Accordingly, as described above, it is hard to form a dot in the 4th area.
As a result, the density attained when the gray level is 4 becomes slightly different from the density attained when the gray level is 3, but greatly different from the density attained when the gray level is 5.
Such a problem also occurs in the case, shown in FIG. 14H, where the gray level is 7. As shown in a plot of density versus gray level in FIG. 16, when the gray level is 4 or 7, the resultant density drops greatly below the gradient of a line and does not attain a density that matches the desired gray level.
Thus, the density does not change smoothly with changes in gray level and, as a result, a poor quality image is produced. When the gray level is 4, the density is particularly low compared with the density attained when the gray level is 7. The above-described problem becomes visually noticeable.